1. Field of the Invention
The invention relates generally to a control unit and control method for a variable valve timing mechanism, and, more specifically to a variable valve timing control that is executed when an internal combustion engine is stopped.
2. Description of the Related Art
A variable valve timing (VVT) mechanism that changes the phase (i.e., crank angle), at which an intake valve or an exhaust valve is opened/closed, based on the engine operating state has been used. Such variable valve timing mechanism usually changes the phase of the intake valve or the exhaust valve by rotating a camshaft, which opens/closes the intake valve or the exhaust valve, relative to, for example, a sprocket. The camshaft is rotated hydraulically or by means of an actuator, for example, an electric motor.
When an engine is stopped, a VVT mechanism usually changes the valve phase toward a target phase that is suitable for next engine starting. For example, Japanese Patent Application Publication No. JP-2004-300924 (JP-A-2004-300924) describes a variable valve control that is executed when an engine is stopped and that is executed over a variable valve mechanism of an engine provided with a three-dimensional cam of which the cam profile is changed in its axial direction and a valve lift amount changing unit that continuously changes the valve lift amount by moving the three-dimensional cam in the camshaft axial direction.
With the variable valve mechanism described in JP-A-2004-300924, when the camshaft needs to be moved in the axial direction when the engine is stopped, the valve lift amount changing unit is driven, and the travel distance of the cam is detected. When the detected travel distance of the cam is shorter than a predetermined value, it is determined whether the period in which the valve lift amount changing unit is driven exceeds a predetermined period. When the period in which the valve lift amount changing unit is driven exceeds the predetermined period, the valve lift amount changing unit is stopped. Thus, it is possible to minimize unnecessary power consumption, thereby increasing the useful life of a battery. In addition, it is possible to minimize overheating due to overload, thereby enhancing the reliability of a system.
However, with the variable valve mechanism described in JP-A-2004-300924, the period in which the valve phase can be changed when the engine stops is limited to the period from when an engine stop command is issued until when the engine actually stops. With a variable valve timing mechanism with which it is difficult to change the valve phase after an engine stops, the amount by which the valve timing can be changed when the engine stops is limited to a relatively small amount. Therefore, there is a possibility that the valve phase when the engine stops does not reach a predetermined valve phase suitable for engine starting, which deteriorates startability when the engine is started next time.